This invention relates to the preparation of diaryl carbonates by carbonylation. More particularly, it relates to the improvement of diaryl carbonate yield and selectivity in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods that employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply "carbonylation" for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is a heavy Group VIII metal; i.e., a Group VIII metal having an atomic number of at least 44, said metals consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof.
The production of carbonates may frequently be improved by including a lead-containing cocatalyst along with the heavy Group VIII metal catalyst. Suitable lead-containing cocatalysts have been described broadly in various patents and publications, particularly in U.S. Pat. No. 5,498,789. Also preferred in general is the use of various halides, as illustrated by tetra-n-butylammonium bromide, as part of the catalyst package. Compounds characterized as inert solvents, such as toluene, diethyl ether, diphenyl ether and acetonitrile, can also be present.
Lead-containing systems of this type have, however, certain disadvantages. In the first place, selectivity of the reaction (i.e., the amount of diphenyl carbonate produced as a percentage of total reaction products derived from phenol) is poor, with various by-products such as biphenols and bromophenols being formed in addition to the desired diaryl carbonates. In the second place, sediments are often formed when lead compounds such as lead(II) oxide are added to other components of the catalyst package, and their presence can result in poor reproducibility of reaction results. In the third place, water is frequently formed by the reaction of lead(II) oxide with the other catalyst components and may adversely affect the activity of the catalyst.
It is of interest, therefore, to develop lead-containing catalyst systems that do not adversely affect catalyst activity.